Polyalkyl aromatic hydrocarbons such as xylene and mesitylene have a plurality of isomers. For example, a mixed xylene obtained from petroleum-reformed oil or cracked gasoline contains 4 isomers of p-xylene, m-xylene, o-xylene and ethylbenzene. Since the plurality of isomers have boiling points close to one another, for example, p-xylene (hereinafter, abbreviated to “PX” in some cases) has a boiling point of 138° C.; m-xylene (hereinafter, abbreviated to “MX” in some cases) has a boiling point of 139° C.; and o-xylene (hereinafter, abbreviated to “OX” in some cases) has a boiling point of 144° C., it is difficult to obtain a desired compound from an isomer mixture by a usual distilling method of carrying out separation by utilizing the difference in boiling point.
Methods of separating a specific aromatic hydrocarbon alone from a mixture of aromatic hydrocarbons containing a plurality of isomers have been developed so far. For example, two types of methods of industrially separating MX from a mixed xylene are known. One method thereof is a known method of using an adsorbent having an affinity for MX, and adsorbing and separating MX alone from a mixed xylene, and for example, the method utilizes the principle of liquid chromatogram and adsorbs and separates MX, which has high affinity for an adsorbent such as zeolite, selectively from a mixed xylene to thereby obtain MX (see Patent Literature 1).
Another method is a method of using hydrogen fluoride and boron trifluoride being a superacid as an extractant, and the method of extracting and separating MX alone selectively from a mixed xylene by utilizing the fact that MX has a higher basicity than the other C8 aromatic hydrocarbon compounds (PX, OX and ethylbenzene). This method has an advantage of having a higher selectivity of MX than the above-mentioned method of using an adsorbent. For example, it is known that when MX is separated from a C8 aromatic hydrocarbon mixture, the selectivity of MX with respect to OX, which is a separation key component, is 2 in a method of using an adsorbent (see paragraph [0037] in Patent Literature 1), whereas the selectivity is 10 in the method of using hydrogen fluoride and boron trifluoride being a superacid and separating and extracting MX. Further, the method of using a superacid as an extractant allows extracting and separating mesitylene alone, which has a peculiarly high basicity, also from an isomer mixture of C9 alkyl aromatic hydrocarbons in addition to a mixed xylene because the difference, if any, in basicity among isomers in an isomer mixture can be utilized.
As a method of using hydrogen fluoride and boron trifluoride as an extractant and extracting and separating MX from a mixed xylene, Patent Literature 2 proposes a method in which a mixed xylene is fed from the central section of an extraction column; liquid hydrogen fluoride and boron trifluoride are fed from the column top section; a diluent is fed from the column bottom section; and countercurrent extraction is continuously carried out under the temperature condition of −20° C. to +30° C. to thereby extract a MX-HF-BF3 complex into surplus hydrogen fluoride and separate MX from the mixed xylene.
Further, Patent Literature 3 proposes that a MX-HF-BF3 complex is thermally decomposed in a decomposition column, and hydrogen fluoride and boron trifluoride are isolated and recycled. The Patent Literature describes that the complex is decomposed at a pressure of 2 to 10 atm and at a column bottom temperature of 131 to 217° C. while hexane as a decomposition auxiliary agent is evaporated and refluxed; and then, MX is withdrawn from the column bottom of the decomposition column, and hydrogen fluoride and boron trifluoride as the extractant are recovered from the column top and recycled.
It is further known that if a specific base is added to an aromatic hydrocarbon-HF-BF3 complex such as a MX—HF-BF3 complex, there occurs an equilibrium reaction of exchanging the complex states between the aromatic hydrocarbon and the base, and thus the utilization thereof enables measuring the relative basicities of individual aromatic hydrocarbons with respect to xylene (see Non Patent Literature 1). Non Patent Literature 1 describes a reaction shown below as a complex-forming reaction.A+HF+BF3→A.H++BF4−
In the above formula, “A” represents an aromatic hydrocarbon; and “A.H++BF4−” represents an aromatic hydrocarbon-HF-BF3 complex.